(U.S. Air Force)

Medium Launch Vehicles for Satellite Delivery

Joseph F. Wambolt

Decommissioned as weapons systems, early ballistic missiles made a successful transition to the U.S. space program, providing vital medium-lift capacity for military and scientific missions.

The intercontinental ballistic missile (ICBM) and intermediate-range ballistic missile (IRBM) programs progressed rapidly in the 1950s and 1960s, spurred by the need to establish a nuclear deterrent. But DOD realized that superiority in space would ultimately prove as important as a long-range strike capability. As a result, many of the medium-lift boosters that came from these ICBM and IRBM programs were enhanced to meet the demanding performance requirements of space-related missions. In particular, many versions of the Thor/Delta and Atlas boosters were modified to fly with various upper stages, such as the Burner, Agena, and Centaur. The Aerospace Corporation played a vital role in these early space-related booster developments, and continues to support the advancement of medium-lift launch vehicle technology.

First Flights

The first Thor IRBM was launched in January 1957, followed by the first Atlas ICBM in June. Within a short time, the first polar-orbiting satellites—including the very successful but highly classified Discoverer series—were launched using the Thor booster and Agena upper stage combination. Atlas orbital missions began in December 1958—just in time to permit the first satellite transmission of a Christmas message from President Eisenhower.

An Atlas E lifts a Navstar/GPS Block I satellite into orbit from Vandenberg in October 1985. (U.S. Air Force)

By mid-1960, Thor had flown more than 90 flights, and various Atlas configurations had flown 55 times; however, success rates for both vehicles barely reached 65–70 percent. NASA, newly formed in October 1958, teamed with the Air Force Ballistic Missile Division to oversee modification of the Atlas D booster for the piloted Mercury spacecraft and later the Titan II booster for the piloted Gemini flights. The Air Force in turn asked the newly formed Aerospace Corporation to provide general systems engineering and integration. Many of the Aerospace engineers on the Mercury-Atlas program came from the Atlas ICBM development project. As a result, Aerospace was able to use its technical expertise to directly influence the success of the piloted Mercury and Gemini missions. Aerospace also became involved in the Atlas standard launch vehicle flights in support of classified Air Force programs.

Upgrading the Atlas

While the Atlas was achieving notable success in the spaceflight area, its usefulness for purely military applications was coming to an end. With the advent of the solid-fueled Minuteman ICBM, the liquid-fueled Atlas E/F vehicles were deactivated as weapons systems in 1965 and shipped to Norton Air Force Base in California for storage. In the next six years, more than 30 of them were flown to support the research efforts of the Advanced Ballistic Re-Entry Systems program (ABRES), which was investigating ways to penetrate Soviet missile defenses. As a result of these research flights, the Air Force Space Test Program discovered the economy of these vehicles, and sought to use them for selected scientific space missions.

From 1970 to 1971, Aerospace recommended several reliability improvements that would foster the use of weapon-grade Atlas E/F boosters for space-related programs. For example, Aerospace pressed for upgrades in electronic part quality, redundancy in hydraulic control systems, and additional environmental testing of critical guidance components. Other improvements included redundant ground guidance computers and a greater emphasis on failure analysis and corrective action (see sidebar, Evolution of the Atlas).

An Atlas II blasts off from Cape Canaveral in February 1992 carrying a Defense Satellite Communications System (DSCS III) payload into a geosynchronous transfer orbit for the Air Force. (U.S. Air Force)

In 1971, the Air Force transferred responsibility for the Atlas E/F assets from the Ballistic Missile Division to the Launch Vehicles System Program Office. But converting these defensive missiles into safe and reliable space launch vehicles was no easy task, and Aerospace focused attention on the need for increased technical oversight. For example, the decommissioned ICBM fleet had been deployed and filled periodically with propellants as part of military training exercises. Consequently, they would need refurbishment and modification to bring them back to baseline condition. The Air Force implemented a program to remove the aging Atlas vehicles from storage and refurbish them on an "as needed" basis. With Aerospace assistance, the Air Force would conduct a formal validation of each booster after the refurbishment was complete. Aerospace worked closely with the Air Force to construct a Launch-Readiness Certification program for every mission flown on Atlas E/F, employing all the Air Force systems engineering policies in effect at the time. Essentially the same approach is used today on Atlas and Delta missions, including the component evaluation and emphasis on failure analysis and corrective action.

The user community grew in the mid-1970s, and Aerospace's general systems engineering and integration responsibilities expanded to include many missions that posed unique requirements for integration and launch vehicle design. The most challenging, for example, was Seasat, which integrated an Agena second stage with the Atlas E/F for the first time and flew from a modified launchpad. The Seasat mission was the heaviest upper stage and satellite combination ever flown on the Atlas E/F, and it necessitated a new approach to structural and flight-control system design and analysis. Likewise, early GPS missions employed a newly developed two-stage spin-stabilized upper stage, which required extensive analysis and testing to ensure that the separation of the spinning stages would be stable.

The Atlas II booster was 2.7 meters longer than the Atlas I and featured more powerful and efficient engines. The Atlas IIAS, shown here, added four solid-rocket boosters to the core Atlas stage. The Atlas II series has achieved 100 percent operational success. (View larger image.)

During these years, Aerospace developed an array of tools for independently analyzing loads, controls, trajectories, and environments. Three-dimensional load analysis was introduced to Atlas for the first time. Original plans to launch an occasional Space Test Program mission grew to include 52 missions, which depleted the inventory of stored Atlas E/F vehicles (including several retrieved from museums). The first of these missions was launched successfully in October 1972. Although 28 successful missions were achieved, three failures involving propulsion prompted a major technical upgrade of the remaining 21 vehicles in 1981, including a complete teardown, rebuild, and hot fire of the Atlas MA-3 engine systems.

The upgrade changes were made to all critical systems of the Atlas E/F booster, including ground checkout equipment and procedures. Aerospace initiated a yearlong independent review to evaluate the changes being made to the remaining 21 Atlas E/F boosters to instill confidence that those missions would be successful. In addition to the engine overhaul program, flight-control and guidance systems, hardware, and test equipment were retrofitted with the solid-state electronics that were being used in the newly manufactured Atlas space launch vehicles. The Air Force funded this upgrade effort and also formed a permanent reliability improvement program to allow for the evaluation and implementation of new ideas as the flyout continued. As a result, all 21 of these upgraded Atlas E/F vehicles successfully placed satellites into their prescribed orbits, serving programs and organizations as diverse as the Defense Meteorological Satellite Program, Global Positioning System, National Oceanic and Atmospheric Administration, Space Test Program, and National Reconnaissance Office. The last such mission was successfully flown in March 1995.

Aerospace support for the Atlas E/F refurbishment and launch program lasted more than 24 years. The Atlas E/F provided an economical (less than $15 million) and reliable booster for many research and development programs and proof-of-concept test flights.

Exploded view of the Delta II rocket carrying a Global Positioning System satellite payload. The PAM, or payload-assist module, is an upper stage used to finalize spacecraft orbits. (View larger image.)

Priority Shifts

In 1972, with the end of the space race, President Nixon decreed that the United States would rely on the partially reusable space shuttle for routine, reliable, inexpensive access to space. This represented a major shift in the national priority: No longer would the United States employ expendable launch vehicles based on the original ICBMs.

With this strategy in mind, DOD and NASA began to launch the last of their expendable launch vehicles and stopped investing in any facilities, infrastructure, and range systems that were not required to support the space shuttle. NASA adapted facilities developed for the Apollo program to support the space shuttle at Kennedy Space Center in Florida, and the Air Force undertook a $4 billion development program to build and certify extensive shuttle processing and launch facilities at Vandenberg Air Force Base in California.

The loss of the Challenger in January 1986 prompted a total reversal of space policy. President Reagan directed federal programs to reinstate an expendable launch vehicle capability for all future routine satellites as well as for DOD satellite programs previously on the space shuttle manifest.

As a result, the Air Force selected the Delta II to launch GPS satellites and the Atlas II to launch Defense Satellite Communication System (DSCS) satellites. These high-priority defense payloads, which had been designed to fly on the space shuttle, were compatible in size and mass with the boost capability of the medium launch vehicles (see sidebar, A Typical Atlas launch). Moreover, the Delta II and Atlas II represented further evolutions and refinements of the original ICBM designs, enabling them to accommodate these satellites.

	The lift capacity for the Delta family of rockets has increased significantly throughout the years. The latest Delta II configuration features elongated graphite-epoxy strap-on solid-rocket motors (SRMs).

Switching Back: Delta II and Atlas II

Acquisition reform wasn't an official policy in 1988, but the Air Force nonetheless sought to streamline procurement by buying launch services from both contractors. In this way, the Air Force would support the contractors' plans to build launch vehicles to a single standard of quality for NASA, the government, and the resurgent commercial satellite market.

The strategy would provide the Air Force with an economical price based on larger production runs while encouraging the contractors to invest in quality processes and manufacturing facilities. Aerospace was contractually permitted to participate in the development decisions and was charged with determining the launch readiness of every DOD-assigned launch vehicle as it progressed through production and launch preparation.

This 1989 photo shows the first launch of a Delta II from Cape Canaveral carrying a Global Positioning System satellite for the Air Force.

Aerospace performed an independent analysis for each booster-payload configuration (such as GPS, DSCS, STP) and did not repeat the analysis unless a major change was made to the launch vehicle or satellite. These analyses emphasized hardware performance evaluation, anomaly resolution, flight data review, and an extensive "pedigree" evaluation of mission-critical components.

This pedigree evaluation has since become a cornerstone of Aerospace launch-readiness assessments. A tedious and labor-intensive process, it entails an exhaustive background check of the manufacture and test history for more than 200 critical components. Aerospace engineers familiar with the design and manufacturing details of the electronic components, pumps, regulators, engines, valves, and explosive ordnance review the critical hardware to ensure that assembly and testing of each part was completed in a satisfactory manner.

Thanks in part to this pedigree evaluation, the Delta II achieved a remarkable success record for the Air Force, prompting NASA to adopt it for space science missions. In 1992, Aerospace began to conduct component pedigree services for NASA Delta II missions as well, and still performs this service, providing confidence that experienced specialists have conducted painstakingly detailed review of the critical hardware flown on each mission.

A Titan II lifts off from Vandenberg Air Force Base in June 2002 carrying a meteorological satellite for the National Oceanic and Atmospheric Administration. Aerospace verified that all critical hardware, software, and mission analyses met requirements for flightworthiness. (U.S. Air Force)

By 1992, 13 GPS satellites were successfully deployed, and the 24-GPS constellation was completed in March 1994 using the Air Force's redeveloped Delta II. The medium-lift launch vehicle program is still compiling an excellent track record, with 11 of 11 Atlas/DOD launches and 37 of 38 Delta II launches achieving ultimate mission success. The only failure of a Delta II involved one of the eight graphite-epoxy strap-on solid motors. This problem was particularly puzzling, because 333 of these solid motors had already flown without incident. Aerospace determined that the failure was caused by undetected damage to several layers of graphite-epoxy fibers in the protective case surrounding the solid propellant. As the protective outer layers of graphite-epoxy fibers failed, the ability of the motor case to contain the pressure of the internally burning propellant decreased, eventually resulting in rupture. As a result, all motors are now given an extensive ultrasonic inspection prior to flight to find hidden damage from handling and manufacturing.

Conclusion

The medium class of Atlas, Delta, and Titan launch vehicles has provided military, civilian, and commercial space programs with hundreds of successful missions. By helping to develop the "best practice" processes now in place, Aerospace made a major contribution to the ultimate success of these boosters. Applying these reliable processes to the new generation of Atlas and Delta evolved expendable launch vehicles will extend the remarkable performance records for these versatile medium-lift boosters.

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